Discussion: Acute disseminated encephalomyelitis (ADEM) and multiple sclerosis (MS) are both inflammatory demyelinating diseases of the central nervous system (CNS). Whereas ADEM is usually a monophasic illness, MS is by definition a multiphasic disease which frequently results in stepwise or steadily progressive deterioration in neurological function. For this reason differentiation of ADEM from MS in a patient with a single clinical episode attributable to CNS demyelination is of prognostic importance.
Certain clinical features may help to differentiate the two conditions. ADEM often produces a widespread CNS disturbance with coma or drowsiness, seizures, and multifocal neurological signs implicating the brain, spinal cord and optic nerves. In contrast, MS usually presents as a monosymptomatic syndrome such as optic neuritis or a subacute myelopathy. ADEM may also present in this way although with some differences. Thus optic neuritis in ADEM is usually simultaneously bilateral whereas in MS it is more often unilateral; myelopathy in MS is frequently partial but in ADEM it is often complete and associated with areflexia. Nevertheless no clinical feature is exclusive to one or other disorder.
The distinction of MS from ADEM on a single scan in these patients might be facilitated if there were reliable means of determining the age of lesions. In MS, lesions would be of a varying age; in most
patients with ADEM, they would all be the same age. Studies in MS using gadolinium- DTPA enhanced MRI show a mixture of enhancing and nonenhancing lesions and there is evidence that enhancement (which indicates an abnormal blood-brain barrier) is a consistent feature of new and active lesions
Given that ADEM is usually a monophasic disease, it might be expected that all lesions would enhance in the acute phase, while none would do so in the chronic phase. Gadolinium-DTPA enhanced MRI may therefore prove useful in distinguishing monophasic and multiphasic disease, but a study of enhancement in ADEM has yet to be performed. certain patterns of MRI abnormality in ADEM which would
be unusual in MS. In particular, there was a pattern in some cases of extensive and relatively symmetric abnormalities in the cerebral and cerebellar white matter, and in one case in the basal ganglia, a rare finding in MS.Serial MRI offers help in differentiating monophasic from multiphasic disease.

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